Keeping abreast with the recent advances in laser technology, studies have been undertaken on methods for converting laser radiation to the thermal energy necessary for recording, for example an image forming method utilizing a laser thermal recording material or a laser thermal transfer printing material for high-speed, high-density, high-image-quality recording or reproduction. Moreover, against the background of the wide-spread use of computers and rapidly developing electronics inclusive of improvements in digital image processing technology, development of the so-called computer-to-plate (CTP) technology for direct fabrication of printing plates from digital data is in progress.
In the technology comprising converting laser light to heat for image recording (laser thermal recording technology), a photothermal conversion material (light-to-heat converter) suited to the wavelength of a laser is used for conversion of absorbed laser light to heat to form an image but unless the laser output is raised to a fairly high level, the thermal energy required for image formation cannot be obtained. Therefore, there is a standing demand for development of a material having a high photothermal conversion efficiency.
The known CTP technology includes, as classified by plate fabrication methodology, the laser light exposure method, the writing method using a thermal head, the method for local voltage application using pin electrodes, and the ink jet method for forming an ink-repellent or ink-receptive layer, among others. The laser light exposure method, in particular, is superior to other methods in resolution and platemaking speed. In this field, therefore, a variety of image-forming techniques are being studied.
Today, compact, high-output, inexpensive semiconductor lasers having an emission band in the near-infrared region (750 nm.about.900 nm) of the spectrum are readily available and can be exploited as exposure light sources in printing plate fabrication.
The plate fabricating method using laser light is either of the light-sensitive type or of the heat-sensitive type. The light-sensitive plate material is available either in the electrophotographic system using an organic photoconductor (OPC) or in the silver salt system utilizing a salt of silver but both materials have the disadvantage that a large-sized, expensive production equipment is required and that the cost of the plate is fairly high as compared with the conventional presensitized (PS) plate. Furthermore, there is the problem associated with disposal of the developer. Therefore, the above-mentioned plate materials have not been commercially implemented as yet.
The heat-sensitive plate material has the disadvantage of low sensitivity as compared with the light-sensitive plate material but has been extensively studied in view of the advantage that it can be handled under the interior (illuminated) conditions and the required equipment may be small and inexpensive.
The heat-sensitive plate materials invariably require the use of the so-called photothermal transducer for the conversion of light to heat.
It is essential that the photothermal conversion material absorb the laser light used, and for an enhanced sensitivity, its ability to absorb the laser light must be sufficiently high.
The light-to-heat converting substance for such a photothermal conversion material includes pigment type substances and dyestuff type substances. A typical pigment type substance is carbon black. As dyestuff type substances, a variety of substances have been proposed, although polymethine dyes are commonly employed. Carbon black offers a broad choice of compatible lasers but has the disadvantage that its ability to absorb laser light is generally so low compared with dyes that it must be used in large amounts. Moreover, a sophisticated dispersion technique is essential.
The dyestuff type substance must have a large capacity to absorb the emission of the semiconductor laser used, high compatibility with the concomitant image-forming component and resin binder, and high solubility in the solvent used.
Polymethine dyes are essentially of the salt type so that the type of solvent that can be used is limited and the compatibility with the image forming component and resin binder is poor.